1. Field of the Invention
The present invention relates to a communication method suitable for application to, for example, a base station and a terminal apparatus in a radio telephone system, and a base station and a terminal apparatus to which the communication method is applied.
2. Description of the Related Art
In a mobile communication such as a radio telephone system or the like, a multiple access is carried out wherein a plurality of mobile stations (subscribers or terminal apparatus) have access to a single base station. In the case of the radio telephone, since a plurality of mobile stations share a single base station, a variety of communication systems have been proposed in order to avoid interference among the respective mobile stations. For example, conventional methods of communication of this type include a frequency division multiple access (FDMA), a time division multiple access (TDMA), a code division multiple access (CDMA) and so on.
According to the TDMA system, each transmission channel allocated to each base station is divided by a predetermined period of time to form a plurality of time slots in one transmission channel, and each of the time slots is allocated to a mobile station to be connected, thereby allowing simultaneous connection to a plurality of mobile stations using one transmission channel.
Further, as another system of communication, the present applicant has proposed a system of communication referred to as a band division multiple access (BDMA) (Japanese unexamined patent publication No. 8-132434 and others). Although the BDMA system will be explained in detail in preferred embodiments of the invention to be described later, briefly, it is a system wherein a plurality of transmission bands having a predetermined number of subcarrier signals arranged at predetermined frequency intervals are prepared; the signal in each of the transmission bands is segmented by a predetermined period of time to form time slots; and a burst signal is transmitted in the form of a multi-carrier signal which is data distributed among said predetermined number of subcarrier signals intermittently at the cycle of a predetermined number of time slots. The BDMA system exhibits very excellent transmission characteristics.
When the TDMA system is applied to a radio telephone system, a terminal apparatus must communicate to a base station in synchronization with a reference timing set by the base station to prevent interference between time slots of each channel. Since the distance between the terminal apparatus and the base station is not constant, even if a signal is transmitted from each terminal apparatus to the base station at the same timing, different propagation delays are caused in the signal transmitted from each terminal apparatus and hence the timing at which the base station can receive these signals varies for each channel.
Thus, there is a need for some process to correct the timing and, for example, a process has been performed wherein the base station detects the amount of delay of a signal transmitted from each terminal apparatus from the reference timing and control data is transmitted to the terminal apparatus to shift the transmission timing in accordance with the amount of delay, thereby performing correction such that reception can occur at a constant timing (a time alignment process).
However, such a time alignment process involves bidirectional data transmission and, therefore, it can be carried out only on communication in progress between a base station and a terminal apparatus. For example, it has been impossible to perform a time alignment process on an access request signal transmitted from a terminal apparatus to a base station to request the commencement of communication (access request).
Under such circumstances, a signal may be transmitted in a burst formed somewhat short in order to avoid problems associated with the transmission of a signal on which no time alignment process has been performed. Further, in the case of such a short signal burst that can not adequately absorb a shift in time caused by propagation delay, conventional methods can not solve the problem associated with time alignment.
In view of such aspects, it is an object of the present invention to allow an access request or the like to a base station to be carried out preferably for communication using an efficient system having a high transmission rate such as a radio telephone system or the like.
According to a first aspect of the present invention, a communication method of carrying out communication using a time slot as a unit includes a first communication step of using a time slot having a first time length when information of a first kind is communicated, and a second communication step of using a time slot having a second time length when information of a second kind is communicated.
According to a second aspect of the present invention, a base station in a cellular system using a time slot as a unit for communication to a subscriber station includes receiving means for receiving a signal from the subscriber station, reception processing means for carrying out a processing by a unit of a time slot having a first time length when general information is communicated and for carrying out a processing by a unit of a time slot having a second time length, which is longer than the first time length, when communication control information is communicated, signal generating means for generating a signal to be transmitted to the subscriber station, and transmitting means for transmitting the signal generated by the signal generating means to the subscriber station.
According to a third aspect of the present invention, a subscriber station in a cellular system using a time slot as a unit for communication to a base station includes reception means for receiving a signal from the base station, reception processing means for processing a signal received by the reception means, signal generating means for generating a signal formed in a unit of a time slot having a first time length when general information is communicated and for generating a signal formed in a unit of a time slot having a second time length, which is longer than the first time length, when communication control information is communicated, and transmitting means for transmitting the signal generated by the signal generating means to the base station.